FACTORS LIMITING THE YIELD 755 



3. Factors Limiting the Yield of Fluorescence 



What process (or processes) cause the abbreviation of the natural Ufe- 

 time of the excited state A of dissolved chlorophyll molecules and prevent 

 a 100% yield of fluorescence? If a single molecule in vacuum absorbs a 

 light quantum, the probability of fluorescence is 100%, since fluorescence 

 is the only way in which the molecule can get rid of excess energy. A tem- 

 porary distribution of this energy between the internal degrees of freedom 

 or even its transformation into chemical energy, by isomerization or dis- 

 sociation, can delay the emission of fluorescence, but cannot prevent it. 

 Sooner or later, the original composition and configuration of the molecule 

 will be restored, excess energy will again be converted into the original form 

 of electronic excitation and emission of fluorescence will take place. 



In condensed phases, on the other hand, the yield of fluorescence is usu- 

 ally, if not always, less than unity. This weakening of fluorescence by 

 the medium has several causes. The presence of a large number of foreign 

 molecules can make both the above-mentioned "fluorescence-delaying" 

 processes — the dissipation of the electronic excitation energy within the 

 absorbing molecule ("internal conversion") and the transformation of 

 excitation energy into chemical energy— partly or completely irreversible, 

 and thus reduce the probability of fluorescence. The proximity of foreign 

 molecules adds new possibilities of chemical utilization of light energy, 

 since the excited molecule can now react with other molecules encountered 

 during the excitation period. \ fourth possibility of energy loss in condensed 

 systems — in addition to (1) internal conversion, (2) chemical reaction 

 within the excited molecule and (3) chemical reaction with molecules of 

 the medium — is (4) bulk transfer of electronic energy from the excited 

 molecule to a foreign molecule close by. Finally (5), if the fluorescent 

 molecules are present in sufficient concentration, "self-quenching," i. e., 

 energy dissipation by interaction of excited with nonexcited pigment mole- 

 cules, may become significant. 



It is important to keep in mind that the interaction of a fluorescent 

 molecule with a given medium can be complex. For example, the presence 

 of the medium may make internal energy dissipation irreversible, but also 

 slower. Furthermore, the general eff"ect of the solvent surrounding the 

 fluorescent molecule may be one of quenching, but the specific effect of as- 

 sociation of certain groups in the medium with certain groups in the fluores- 

 cent molecule may be one of "protection", i. e., stimulation of fluorescence, 

 and so on. 



We will now consider in more detail the five ways in which fluorescence 

 can be affected by the mutual closeness of molecules in condensed systems. 



